Arctic’s hydrology during global warming at the Palaeocene-Eocene thermal maximum. Mark
Pagani, Nikolai Pedentchouk, Matthew Huber, Appy Sluijs, Stefan Schouten, Henk Brinkhuis,
Jaap S. Sinninghe Damsté, Gerald R. Dickens, and the IODP Expedition 302 Expedition
Scientists.
It is generally accepted that n -alkanes in the range of C
25
to C
33
, exhibiting a strong oddover-even carbon chain predominance, are derived from higher plant leaf waxes
1-6
. However, a single study (Zegouagh et al., 1998)
7
of modern Arctic sediments from the Laptev Sea challenges this common interpretation, and instead suggests these compounds are of algal origin, rather than higher plants. This study is at odds with our interpretation that the longchain n -alkanes with a strong odd-over-even carbon number predominance in the ACEX core are derived from terrestrial plants from the continent, and if valid, challenges our interpretations of isotopic trends.
Zegouagh et al. (1998)
7
interprets the origin of n -alkanes from only three surface sediments collected in the delta of the Lena River and adjacent areas of the Laptev Sea. They find longchain n -alkanes in relatively low abundance with an odd-over-even predominance (with a decreasing carbon preference index from the delta to the sea). They conclude that these compounds derive from algae on the basis of several criteria:
(1) No evidence for macromolecular organic matter derived from higher plants.

The absence of pyrolysis products, such as phenols, characteristic of lignins and cutins from terrestrial sources, is one criteria used to argue against a terrestrial origin for long-chain n alkanes from the Laptev Sea. However, leaf wax lipids in regions distant from terrestrial vegetation are primarily transported by aeolian transport; a process that excludes the bulk of plant tissue. Therefore, the absence of macromolecular organic matter cannot be used as proof as evidence against higher plant sources for long-chain n -alkanes.
(2) “Absence” of long-chain fatty acids of terrestrial origin and selective degradation of fatty acids
The hydrocarbon fraction of the Laptev Sea samples comprise a small amount of the total lipid extract (0.02%), suggesting very small contributions of n -alkanes for these sediments (i.e.,
Table 1 of Zegouagh et al., 1998) 7 . In a typical leaf, n -alkanes are always present and represent
10-90% of the lipid fraction. This is not the case for fatty acids, where in some instances longchain fatty acids are absent (Fig. S1). Thus, the absence of long chain, even-carbon number fatty acids is not necessarily indicative of an absence of terrestrial input. Given that fatty acids in the C
14
-C
22
represent the majority of the acid fraction in sediments analyzed from the Laptev
Sea 8 , and the fact that most marine organisms do not make saturated hydrocarbons, we would expect to find a lipid profile similar to that described in Zegouagh et al. (1998)
7
for a system dominated by algal production and aeolian transported n -alkanes. That is, we would anticipate that molecular distributions would be dominated by low molecular weight fatty acids with a small contribution of n -alkanes with terrestrial characteristics.
Importantly, Zegouagh et al. (1998) 7 use the absence of even-carbon numbered fatty acids in the range of C
24
and C
34
as the primary evidence for a non-terrestrial origin for their n -

alkanes. However, their absence cannot be used as evidence against terrestrial input, but the presence of even-carbon number fatty acids helps confirm the existence of terrestrial input. In contrast to the findings reported by Zegouagh et al. (1998)
7
, the most dominant compounds in the ACEX PETM sediments are even-carbon numbered fatty acids ranging from C
24
to C
34
with the C
28
homologue dominating. This provides clear and supporting evidence that our PETM n alkanes are terrestrial derived.
( 3) Long chain n-alkanes in cultures of marine algae
Zegouagh et al. (1998)
7
acknowledge that most algae produce short chain n -alkanes (e.g., n -
C
17
). They also note that the presence of long chain n -alkanes has been observed in some algal species, but with no odd-over-even predominance. Zegouagh et al. (1998)
7
cite two articles that suggest that some algae produce long-chain n -alkanes with an odd-over-even predominance 9,10 .
However, the observations in these cited studies have never been confirmed and are likely due to contamination
11
. For example, a set of >100 unialgal cultures of marine diatoms
12
have been studied and long-chain n -alkanes with a strong odd-carbon number predominance have never been observed in any culture (Sinninghe Damsté et al., unpublished results). In conclusion, the scientific basis for the interpretation that (some) marine algae produce long-chain n -alkanes with a strong odd-carbon number predominance is at best weak, and perhaps not valid.
(4) The possibility that long-chain odd n-alkenes from the freshwater algae B. braunii are the source of the long chain n-alkanes
Zegouagh et al. (1998) 7 suggest that the source of odd-long-chain n -alkanes derives from

diagenetic reduction of alkenes, potentially produced by B. braunii.
Although B. braunii produces n -alkenes, we stress that B. braunii is a freshwater algae. The pathway of alkene reduction is not completely understood. Polyunsaturated alkenes are labile and rarely preserved in modern sediments. Reduction of alkenes could result in the production of alkanes or competed degraded. Therefore, it is possible that the reduction of alkenes could lead to the profile observed in the Laptev Sea
7
. However, this supposition is highly speculative and cannot be substantiated.
(5) Carbon isotopic compositions suggest long-chain n-alkanes derive from a marine source.
C
3
plants have a fairly wide range of isotopic values, ranging from -20‰ to -35‰. More negative isotopic values occur in closed canopies where the
 13 C of atmospheric CO
2
is relatively depleted in
13
C due to respiration of organic carbon. C
3
plants enriched in
13
C are typically found in more water-stressed environments. Further, n -alkanes will be more depleted in
13 C relative to bulk leaf by ~3 to 5‰. Most n -alkanes measured in the Zegouagh et al.
(1998) 7 article are in the range of -28 to -32‰. However, lower molecular n -alkanes (in the range of C
16
to C
24
) were measured in only one sample. In general, high-molecular weight n alkanes typical of terrestrial plants have
 13
C values very similar to those reported in Zegouagh et al. (1998)
7
. In a study of organic carbon and n -alkanes from surficial sediments and suspended matter in the Arctic Ob and Yenisei Rivers,
 13
C values of n -alkanes from terrestrial sources averaged -26.8‰ and -28.1‰, respectively
13
. These isotopic values parallel the results of Zegouagh et al. (1998)
7
and strongly support an interpretation that the n -alkanes in the
Laptev Sea are terrestrial, and not algal, in origin. Indeed, the study of Arctic Ob and Yenisei
River sediments directly apply n -alkanes with odd-over-even predominance as a proxy for

terrestrial plant (leaf wax) input for these Arctic sediments. This is a typical proxy approach in studies devoted to carbon cycling in the Arctic Ocean 14 .
It is interesting to note that the short-chain n -alkanes of presumed algal origin (C
16
- C
18 with C
18
dominant) in Zegouagh et al. (1998)
7
also have rather
13
C-depleted signatures (-26 to -
30‰). Zegouagh et al. (1998) 7
argue that isotopic similarities between short- and long-chain n alkanes support an interpretation of a common source. However, these short-chain n -alkanes can have both algal and bacterial sources 15,16 , with even-chain length n -alkanes in the range of n -C
14
to n -C
22
generally attributed to bacteria
17
. Several explanations can account for the depleted signatures of these low molecular weight n -alkanes, including algae growth under elevated CO
2
(due to cold water temperatures and greater CO
2
saturation), bacterial components utilizing isotopically depleted inorganic carbon (grown at depth in the water column), or bacterial reworking of higher molecular weight nalkanes 6,18 .
In conclusion, the evidence and arguments presented by Zegouagh et al. (1998)
7
for an algal origin for long-chain n -alkanes with an odd-over-even predominance are not robust. Further, n -alkanes from the Palaeogene Arctic have substantially higher carbon preference indices
(higher values increasingly support terrestrial origins) than those reported in Zegouagh et al.
(1998)
7
. Finally,

D and
 13
C values of the PETM short-chain and long-chain n -alkanes are substantially different from each other and show a different evolution with time–strong evidence that they derive from different sources.
The collection of data measured for the top ~50 cm of Core 32X reveals complex

sedimentary characteristics due to severe drilling disturbance (Fig. S2). One relatively negative
 13
C
TOC
value coupled with abundant Apectodinium spp. suggests that this interval represents
PETM-aged material. However, the top two samples of the
 13
C
TOC
curve indicate an influence of late Palaeocene strata. Samples evaluated horizontally across the core, but remaining at the same depth, give contradictory results. These aspects indicate that drilling disturbance resulted in an inhomogeneous mixture of late Palaeocene and PETM material at the top of Core 32X. In this regard, the leaf wax n -alkane isotope data are interesting. For example,
 13
C values of n alkanes (
 13 C nC27/29
) gradually become more negative upcore, confirming PETM material.
Further, hydrogen isotopes (

D nC27/29
) show the most D-enriched values of the whole succession in this interval. If these values also represent a mixture of pre-PETM and PETM material, this suggests that the
 13
C and

D values of leaf waxes from the original lowest part of the PETM are likely more
13
C-depleted and D-enriched, respectively.
Hydrological Cycle: Model-Data Comparison
To develop a more complete interpretation of the data presented in this study and that of
Sluijs et al. (2006) 19 , we integrate model results and proxy data interpretations of hydrological state for the Palaeocene and earliest Eocene (or PETM where available). Despite the acknowledged weaknesses of models in reproducing early Palaeogene temperatures, we believe that analysis of climate model results and comparison with proxies is a useful process because it provides a physical framework for proxy interpretation where proxy constraints are missing or open to multiple interpretations. In this section we provide a climate model context for our interpretation of proxies for the hydrological cycle and demonstrate that the model compares well with data. We begin with a theoretical framework.

Theory: To determine whether an increase in the hydrological cycle occurred using paleoclimate proxies, the location of proxy sites within the large-scale evaporation (E) and precipitation (P) distribution is necessary. A ‘more intense’ hydrological cycle as commonly defined in atmospheric dynamics
20
refers to increased water vapor transport from low to high latitudes, or in other words, a stronger divergence of water vapor fluxes. In steady state, stronger divergence requires an intensification of E in net E zones and a counterbalancing increase in P in net P zones, leading to an increased meridional gradient of E-P
21
. In steady state, provided that E>P equatorward of the subtropical margins (e.g., the work of Ziegler et al.
(2004)
22
suggests this has been the case since the Permian) there must be net P averaged over the extratropics. Thus, we expect to find an apparent drying of the subtropical regions to be consistent with an increase in the vigor of the hydrological cycle with a compensating moistening in high latitudes.
The issue that determines the high-latitude water balance is not a local argument (i.e., that increases in temperature must lead to an increase in evaporation; a true statement) but a global argument: that as long as high latitudes have cooler temperatures than the subtropics, we expect the source of water that drives the atmosphere’s hydrological cycle to be from the low latitudes and the sink to be in the high latitudes. This argument is most accurate in a zonal average sense, but is likely to apply generally because zonal advection of water vapor is rapid and maintains a fairly homogenous water vapor distribution in the zonal direction
23
.
Model and experiments : The results provided below are from fully coupled climate model simulations performed with boundary conditions suitable for the early Palaeogene. The model

is the Community Climate System Model v. 1.4 developed at the National Center for
Atmospheric Research 24 . The boundary conditions (topography, bathymetry, vegetation) and initial conditions are described in Sewall et al. (2000)
25
Huber and Sloan (2001)
26
, and Huber et al. (2003)
27
. We show results from two simulations: one with specified carbon dioxide concentrations set at 1120 ppm (denoted PETM-like) and another at 560 ppm (denoted pre-
PETM). The pre-PETM simulation’s ocean circulation is described in Huber et al., (2003;
2004) 27,28 and the tropical ocean behavior is described in Huber and Caballero (2003) 29 . The
PETM-like simulation has not been previously described in depth—it is qualitatively similar to the pre-PETM simulation albeit with substantial high-latitude and deep-ocean warming (deepocean and high-latitude oceans are approximately 11°C warmer than modern values).
Given theoretical constraints, we expect high-latitude soils to have been relatively close to saturation at all times, with increases in precipitation largely balanced by increases in runoff.
This can easily be seen, since, in steady state:
E-P = R where R is runoff. Provided that P>E as warming occurs, we expect increases in P to be balanced by changes in R. Soil moisture plays the role of a small storage term in the non-steady state balance, but does not appear in the steady-state balance other than implicitly through its potential feedbacks on E or P.
E-P, Relative Humidity, and Soil Moisture Distribution Results: Comparison of pre-PETM to
PETM climate model simulations indicates a robust evaporation minus precipitation (E-P) global pattern with minor-to-no shifts in the regions experiencing net P or net E (Fig. S3). In both simulations, higher latitudes experience strongly net P, whereas low-to mid-latitudes

experience strongly net E. One might argue that high-latitude evaporation is not strong enough in these simulations because of the model’s cool bias 30 . While this is likely true to some extent, we argue that the appearance of P>E at high latitudes is not simply an artifact of the cool bias of the model, but more of a reflection of the basic physical balance previously described.
Comparison with fixed SST simulations, with SSTs specified to be close to proxy estimates for the PETM (thus obviating part of the cool bias), yield very similar E-P distributions. This robust spatial E-P pattern is consistent with proxy records 22 . In detail, the E-P pattern becomes more complex in regions strongly affected by topography such as the western interior of North
America, where a orographically driven ‘rain shadow’ is adjacent to a monsoonal plume of precipitation that emerges from the Gulf of Mexico as a summer season low-level jet
25
.
Although this variability is expected in mid-latitude regions with strong topographic variation, we anticipate the Arctic region to be a less complex environment within which the hydrological balance can be explored.
Relative humidity is essentially invariant in these simulations (Fig. S4); a result generic to climate models and widely considered to be an emergent property of the climate system
31
.
Model derived soil moisture distributions are only slightly sensitive to warming, and in general in both simulations indicate soil moistures are > 60% (Fig. S5) at mid- and high latitudes. In other words, the model—in agreement with the basic physical arguments made above—place high latitudes in the net P dominated regime in which nearly saturated conditions prevail.
The model shows a mild increase in net E over the subtropical oceans, and a mild increase in net P at high latitudes (Fig. S3) with global warming, which reflects the enhanced hydrological cycle of these simulations 20 . As noted above, drying or moistening of a given region without the appropriate large-scale context does not provide information about the

hydrological cycle (as defined here). Therefore, in order to compare model predictions with proxy interpretations, we focus on regions where we expect net P to dominate (higher latitudes) separate from region where we expect net E (low- to mid-latitudes).
Palaeocene high latitude records: The Palaeocene is commonly considered to be one of the wettest intervals in Earth history. Some of the most extensive and thickest coal deposits of the
Cenozoic 22,32 , in western North America as well as Alaska and Northern Canada 33 ( for a global map see Ziegler et al., 2004
22
or www.scotese.com) were deposited in the Palaeocene. Coals form from peats, which have a narrow tolerance for unsaturated conditions
34
and are considered excellent indicators of conditions in which P dominated over E, leading to nearly saturated conditions
35
. Crucially, many of these coals, such as the “Fort Union” and “Ravenscrag” coals distributed across a large area of the northern Rockies and Northern Great Plains, are characterized by low-sulfur values. As such, they are especially indicative of year-round fully saturated conditions during their formation and formed from raised mires
34
. Such coals are widespread and also occur at other high-latitude locations, such as the late Palaeocene
Sagavanirktok Formation in Alaska
36,37
.
In addition to the widespread record of perennially saturated conditions derived from the presence of coal deposits, other independent quantitative and qualitative proxies for surface paleoenvironmental and hydrological conditions are routinely interpreted as showing that the late Palaeocene was variously humid, mesic, and subtropical-to-warm temperate
38-42
. Highlatitude vegetation during the Palaeocene was polar deciduous (mixed coniferous and angiosperm) forests. The climate regime is widely referred to as “mesothermal, humid” 43 . In regions where reasonably complete records exist, such as in the Canadian High Arctic

(Ellesmere Island and environs), coastal regions were swamps and wetlands, conditions further inland were floodplains and braided stream plains continuing into inferred upland forests 43,44
Conditions such as these are widely reflected throughout high-latitude paleoclimate records
41
.
Other extratropical regions were characterized by evergreen broad-leaved forests with a
“mixed” subtropical character 41
. Records of massive bursts of highly weathered clays at high latitudes
45-47
can also be considered evidence of major increases in high-latitude hydrological cycling during this interval, although the exact timing and interpretation of these records is not always exact
48,49
.
Low- to mid-latitude records: High resolution records for the Palaeocene
50,51
provide paleobotanical estimates that late Palaeocene (Clarkforkian) mean annual precipitation in
Wyoming was about 130 cm/year (characterized as ‘humid’ subtropical rainforest). Within this region, work focused on hydrologic and temperature variability; both temporally and spatially
52-55 . An early Palaeocene ‘rain forest’ was recently discovered in Colorado 55
.
Paleoprecipitation estimates for this site are roughly double the value for contemporaneous sites located nearby—a result that may be due to orographic influence
56
. This distribution is consistent with our model results, but more importantly, it highlights the potential for strong regional variability in hydrological fields in this region. Thus, sites in western North America are difficult to interpret in that regard because the inferred water balance of the region is likely to be strongly dependent on topography. With that caveat in mind, we note that most of
Wyoming is a region of strong net P in these simulations, immediately adjacent to regions of net E. This pattern of variability over small scales is grossly consistent with previously noted apparent moisture gradients in the region
53,54
. Model-predicted precipitation is ~150 cm/year in

Wyoming, increasing slightly with warming (~10 cm/yr). These values compare favorably with proxy estimates 51 .
Other information is available from floral, clay, and isotopic record. The kaolinitic pulses that have been interpreted as indicating more humid conditions during the PETM at other mid- to high-latitude sites
45,47
do not occur in Wyoming despite the tight scrutiny that has been applied to the region and its excellent, high-resolution records. Other sites
57,58
experience moderate net evaporative conditions in the model, consistent with their location at the margins of the subtropics, under the subtropical high pressure zones. Northern Spanish and Paris Basin sites
59,60
, are net E regions (as inferred from paleoclimate proxies and predicted by the model), but are near a transition region between net E and net P.
High-latitude continental regions in our simulations experience strongly net P conditions in qualitative agreement with proxies results for both the Palaeocene and early Eocene that indicate ‘humid’, often swamp-like, conditions at higher latitudes. While quantitative estimates of high-precipitation values exist for some regions and qualitative arguments for ‘humid’
Palaeocene conditions are widely available, including high-latitude regions
40,41
, these do not directly constrain important hydrological parameters such as boundary-layer relative humidity and soil moisture that make up the crucial parameters for the carbon isotope discrimination of terrestrial plants
52
. The presence of thick, low sulfur coal deposits is a strong indication of nearly saturated conditions over large areas during the Palaeocene, but we seek to supplement this information by objectively making relative humidity and soil moisture estimates from what is observable from the paleobotanical record. To this end, we derive minimum relative humidity and soil moisture values by comparing the modern distribution of vegetation biomes to modern relative humidity and soil moisture distributions. This involves answering the

question: When paleobotanical conditions are ‘humid’, what quantitative information does it convey?
In the next section we calibrate qualitative proxies for Palaeogene hydrological conditions in order to make quantitative characterizations. As we show, calibrated proxy records provide good evidence of moist conditions in the late Palaeocene (pre-PETM), consistent with these model results. Such results provide more confidence in applying these model results to understand paleoenvironmental conditions where other proxies remain unconstrained.
Modern calibration for hydrological conditions: What does ‘humid’ mean?
We have analyzed soil moisture (Figs. S6, S7), relative humidity (Fig. S8), and vegetation distribution data from the European Center for Medium-Range Weather Forecasting 40-year reanalysis project (ERA40) as described in Hageman et al. (2005)
61
. The data are available from the National Center for Atmospheric Research (at http://dss.ucar.edu). While no humidity or soil moisture data set is perfect—and the lack of an agreed upon ‘best’ global soil moisture data set is well-established
62,63 —the ERA40 data have the benefit of being high resolution (~1.25°x1.25°) and global, driven by data where available and being physically consistent elsewhere. Since soil moisture data are especially subject to large uncertainties, we have verified our major conclusions by comparing them with published global soil moisture analyses
61
and by choosing our thresholds in a conservative manner. The goal is simply to be able to turn the typical characterization of “humid, subtropical” by geologists and paleobotanists into a quantitative constraint on hydrological conditions. Focusing on gross patterns should be sufficient to our purposes.
In regions with temperature comparable to those estimated for the early Palaeogene, dense

forests are associated with soil moistures of approximately 0.6; open forests (in other vegetation classifications these grade into wet savannahs) have a volumetric soil moisture fraction of about 0.5 (Fig. S6). In semi-arid zones with vegetation, soil moisture fraction in the warm regions under consideration is even lower, ~0.3. The distribution of soil moisture at this arid end of the vegetation spectrum is extremely sparse, in fact, most regions with soil moistures at or below 0.3, are un-vegetated in ERA40. Ziegler et al. (2004)
22
and Scotese
(www.scotese.com) have mapped out aridity sensitive paleoclimate proxies and these only occur in their reconstructions within the subtropics (i.e., roughly equatorward of 30°), which is consistent with clay records interpretable as aridity signals in the Tethyan region
56,58,59
, as well as our soil moisture results.
Although there was a fair amount of variability in the vegetation distribution during the late
Palaeocene, in general the mid- and high latitudes were well vegetated with forests. During the late Palaeocene more ‘subtropical’ or ‘paratropical’ elements appear at mid- to high latitudes 41
.
However, conditions in these regions never reach the intensely thermophilic and apparently
‘moist’ conditions of the Early Eocene Climatic Optimum. By choosing the evergreen broadleaf vegetation type and focusing our attention on regions of modern tropical rainforests we can formulate a ‘wet’ likely soil moisture end-member of ~0.8. We take the average dense forest number within the tropics as a likely dry end member of ~0.6. These average values can be checked against other global soil moisture data sets
54
, as well as field observations in forested regions with tropical, subtropical, and warm temperature affinities
22,65,66,67
. We find that the 0.8 to 0.6 range covers most published estimates of mean soil moisture in these regions
(Fig. S7).
A similar analysis for relative humidity derived from the bottom-most model layer from

ERA40 yielded best estimates for average humidity in warm temperate to tropical forests of 60-
80% (Fig. S8). Whereas some field observations of analogous forests indicate soil moistures in the 25-45% range
67,68
analysis of relative humidity in these regions reveals that conditions are perennially above 70%. The dual criteria of relative humidity <60% and soil moisture <45% is especially useful because it eliminates all well-forested regions from the ERA40 data set (and is similarly restrictive when applied to field data) and from an analysis of field observations in analogous regions.
Given this analysis, we find that the presence of dense, warm temperate-to-subtropical forests that characterize the Late Palaeocene mid- and high latitudes, can be used to coarsely, but quantitatively, estimate the hydrological state. Our best conservative estimate, based on the paleofloral evidence and the widespread occurrence of low sulfur coals, is that conditions were
~60-70% soil moisture and ~70% relative humidity during the late Palaeocene. The calibrated estimates agree with our model results.
Hydrological setting and implications for the carbon cycle
Consequences of this hydrologic characterization are important for the interpretation of the role of soil moisture and boundary layer relative humidity in changing the carbon isotopic discrimination of terrestrial plants (CIF) during the PETM. As demonstrated by Bowen et al.
(2004)
52
, differences in the magnitude of the negative carbon isotope excursion (CIE) during the PETM between the ocean and the terrestrial biosphere could be driven by changes in soil moisture and ambient atmospheric leaf moisture. A change in the CIF can be negative, positive or zero, depending on assumptions regarding the beginning and final values of environmental moisture. This model was used to explain why soil carbonates appear much more
13
C-depleted

relative to ocean records. It is also important to understand that in order for this model to be an effective explanation, specific initial soil moisture and humidity conditions are required. The relationships embodied in Figure 3 of Bowen et al. (2004)
52 are extremely nonlinear and the final differential fractionation is strongly dependent on the choice of initial (and final) relative humidity and soil moisture values.
Fundamental to the explanation argued by Bowen et al. (2004)
52
are the initial soil moisture and humidity conditions. For example, an offset between our n -alkane δ 13 C CIE (-4.5 to -6‰) and typical deep ocean CIE (~-2.5 to -3‰) requires that the terrestrial CIF increased by 2 to
3‰ during the PETM. Given an increase of at least 5˚C (based on Tex
86’
) in the Arctic, and an associated reduction in CIF, the CIF during the PETM would have to increase by 5 to 6‰ due to increases in soil moisture and humidity. To accomplish a CIF increase of this magnitude,
Bowen et al. (2004) 52 assume that pre-PETM soil conditions are at or lower than 30% and relative humidity is at or less than 60%. Lower soil moistures would allow for a higher initial relative humidity and vice versa. The key determinant being that soil moisture must be <40% for their model to be an efficient explanation for our results from the Arctic. However, after consideration of paleoenvironmental proxies (supported by model results), we do not find it likely that our high-latitude site was semiarid to arid (i.e., soil moistures of 0.3 or less). Thus, soil moisture and relative humidity values chosen as initial conditions by Bowen et al. (2004)
52 are not appropriate for the Arctic. Further, our best guess values as initial conditions leads to
PETM CIF values that do not explain the discrepancy between the marine and terrestrial records.
High latitude salinity

The enhanced hydrological cycle produced in these simulations
20
yields low salinity highlatitude ocean conditions (Fig. S9). The Arctic Ocean (which is cut-off from the global ocean in these simulations) has surface salinities of 22-19 psu, and adjacent high-latitude ocean waters in the North Pacific are as low as 24 psu.
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SI Figure Captions
Figure S1. Lipid distributions from Betual pubescens (Schouten and Sinninghe Damsté, unpublished).
Figure S2. Indicators for mixing of latest Palaeocene and PETM sediments in the top of Core
32X. Core picture (IODP Section interval 302-4A-32X-1, 0-60 cm; Backman et al., 2006) showing disturbed interval. B. Geochemical and palynological results showing that the top of
Core 32X represents a mixture of late Palaeocene and PETM sediments.
Figure S3. Evaporation-Precipitation. Mean annual evaporation minus precipitation for the PETM-like case is shown in (a) and for the pre-PETM case in (b). Net precipitation dominates in both cases at high latitudes, and the zones of net evaporation shift little. In
(c) a difference map is shown, demonstrating slightly higher net precipitation at high latitudes in the PETM case.
Figure S4. Relative Humidity. Conventions are similar to Figure S2. Relative humidity is nearly invariant between cases and is uniformly near saturation at high latitudes.
Differences are statistically indistinguishable from zero.
Figure S5. Soil Moisture. Mean annual volumetric soil water fraction is shown pre-PETM case
(top), the PETM-like case (middle). The difference is shown in the bottom figure.

Figure S6. ERA40 annual mean soil moisture fraction. Regions with soil moisture fraction less than 0.45 have been colored grey (note: 1 = fully saturated). Notably the grey regions highlight most of the desert and semi-arid regions on Earth today. Tropical and subtropical rainforests that might be considered an analogue to the late Palaeocene have soil moisture values between 0.5 and 0.85. Mid-latitudes (for example the warm temperature zone of the Southeast of the United States, which may be also be a
Palaeocene analogue) have values of ~0.6.
Figure S7. Zonally and annually averaged soil moisture fraction from ERA40. In the grey curve, soil moisture for model grid cells with evergreen and deciduous, broadleaf and needleleaf tree upper level vegetation types have been considered. In the red curve, the soil moisture for ‘mixed’ and ‘interrupted’ forest types were sampled. In the green curve, soil moisture in the semi-arid vegetation type was sampled.
Figure S8. Annual mean nearest-surface relative humidity derived from ERA-40.
Seasonal averages produce patterns that do not change the conclusions of this paper.
Figure S9. Salinity transect. Model-prognosed Arctic ocean salinity transect for the
PETM case in units of psu. The transect is taken along a line between the latitudes and longitudes indicated on the figure.